The invention relates to a vehicle steering apparatus for motor vehicles with a drive to support the steering apparatus which is connected to a steering gear via a gear arrangement with an output shaft.
This type of drive with a transmission is used in vehicle steering apparatuses for electric power assisted steering and for rotational speed superimposition devices.
In electric power assisted steering, an electric motor with downstream reduction gear injects a servo force into the steering shaft or the transverse rack of the steering system as a function of the turn of the steering wheel. This reduces the effort required at the steering wheel to steer the vehicle. It is also possible to use electronic control means to provide the assistive power at the right time and based on the desired behavior, for example, a correspondingly greater amount when the vehicle is stationary. Such electric power assisted steering is therefore increasingly used today.
Another important field of application for such electric motor transmission arrangements in steering systems is particularly the rotational speed superimposition device with auxiliary drive for a steering system for non-railbound motor vehicles, which superimposes the rotational speeds of the auxiliary drive and the steering intervention by the driver at the steering wheel and transmits this to the steering motion of the wheels.
Rotational speed superimposition devices of this type, to the extent that they are used for steering systems, are also referred to in the art as devices for superimposing the angle of rotation. Superimposition of the rotational speed and superimposition of the angle of rotation are synonymous.
A number of systems for forming electrical auxiliary drives with gear arrangement for use in the aforementioned arrangements are known in the art.
Rotational speed superimposition transmissions are often realized by means of planetary or worm gears. For example, the German publication DE19823721A1 discloses such a rotational speed superimposition device. Here, a steering wheel drives a housing, which accommodates the gearing of a ring gear of two planetary gear sets. An electric motor arranged in the housing drives the sun gear of the first planetary gear set. The planet carrier of the first planetary gear set drives the sun gear of the second planetary gear set. The planets of the second planetary gear set are supported on the ring gear of the housing and the planet carrier is connected to the output shaft. In the embodiment shown, the drive of the sun gear of the first planetary gear set is realized directly by the rotor of the electric motor. By controlling the electric motor accordingly, the desired superimpositions of the rotational speeds can be realized. This prior art is solution has some significant disadvantages, however. Gears of this type consist of many different components. Two ring gears or planetary gear sets are required. Producing the individual parts is highly complex and costly. The plurality of gear tooth engagements requires great accuracy. The inherently high noise of such gear systems is undesirable in this application. Moreover, the driver must rotate the entire apparatus, including the motor housing when steering. A further disadvantage is the complex electrical power coupling for the electric motor.
The prior art of DE19852447A1 proposes a solution for speed superimposition where an electric motor is coupled to a superimposed planetary gear set by a worm drive. Here, the gear unit is fixed to the vehicle body. This solution, too, has a number of drawbacks, however. The unit requires many individual parts, which are complex and costly to manufacture, and the manufacturing tolerances required are correspondingly high.
The publication WO 2006/072186 A1 proposes a rotational speed superimposition device which is already relatively compact and has fewer components. The superimposition gear unit consists of a toothed disk connected to an input shaft linked to the steering wheel and an additional toothed disk connected to the output shaft. The two toothed disks each have a different number of teeth and an additional gear meshes with the teeth of both. This gear is mounted eccentrically to a rotor and is rotatable about its own axis. The rotor is driven about the longitudinal shaft axis by an auxiliary drive. The transmission ratio can be adjusted by selecting a different number of teeth.
A drawback of all the aforementioned examples from the prior art is that their operation produces a substantial amount of undesirable noise.